Properties of PSG and BPSG films make them useful in a variety of semiconductor technology applications. Since the properties of these films which make PSG and BPSG useful in a given application may depend on the phosphorus concentration, it is important to be able to establish this concentration. It would be optimal to control the phosphorus concentration during processing, but concentration control during processing is not always possible. In quality control applications, therefore, a fast, nondestructive method of measuring the phosphorus concentration in an integrated circuit processing line is often desirable.
There are a variety of analytical techniques which have been utilized for measuring the phosphorus concentration in PSG and BPSG films, as reviewed in A. C. Adams and S. P. Murarka, "Measuring the Phosphorus Concentration in Deposited Phosphosilicate Films," 126 J. Electrochemical Society 334 (1974). These include: direct chemical analysis, infrared spectroscopy, electron microprobe, neutron activation analysis, x-ray fluorescence, Auger analysis, etch rate variation and diffusion techniques. Chemical analysis, which is the most direct method, is very tedious and is a destructive method. Chemical analysis has typically been used to establish an absolute calibration for other techniques. Other techniques (electron microprobe, Auger analysis, neutron analysis, and x-ray techniques) require relatively sophisticated equipment. Sophisticated equipment adds to the cost of analysis through higher equipment costs as well as additional operator time and training. For frequent, routine analysis, the simpler and less costly techniques (infrared spectroscopy and x-ray fluorescence) have an advantage, provided of course that the precision and accuracy are acceptable. Infrared spectroscopy has previously been used for measuring the phosphorus concentration in PSG and BPSG films.
Previous infrared spectroscopic methods of phosphorus concentration in PSG films are generally disclosed in R. M. Levin, "Water Absorption and Densification of Phosphosilicate Glass Films," 129 J. Electrochemical Society 1765 (August 1985) and A. C. Adams, VLSI Technology, S. M. Sze. Editor, Chapter 3 (McGraw-Hill, New York 1983).
These methods for measuring the phosphorus concentration in PSG films typically involve the use of a double beam spectrophotometer. The PSG film is deposited on a silicon wafer and the wafer is placed in the appropriate beam. A silicon wafer matched to the silicon wafer on which the PSG film is deposited or air, may be used as a reference. The chosen reference is used in the reference beam. Next, a spectrum (either transmission or absorption) of the PSG film is taken from somewhere between 250 to 4000 cm.sup.-1 Next, the amplitude of the P.dbd.O band at about 1316 cm.sup.-1 is measured. This amplitude may then be used with a calibration curve which relates the P.dbd.O band amplitude to actual phosphorus concentration. A calibration curve is constructed by determining the P.dbd.O band amplitudes for samples used as standards and plotting the amplitudes against the actual phosphorus concentrations of these known standards. The actual phosphorus concentrations of these standards are typically measured by chemical analysis techniques.
Alternately, the amplitudes of the P.dbd.O band at about 1316 cm.sup.-1 and either of the Si--O bands at about 818 cm.sup.-1 or at about 1080 cm.sup.-1 are measured. A ratio of these amplitudes is calculated, with the P.dbd.O band amplitude as the numerator and the chosen Si--O band amplitude as the denominator. Use of this amplitude ratio obviates the need to know the film thickness. Next, a calibration curve is constructed by determining this amplitude ratio for samples used as standards and plotting these ratios against the corresponding actual phosphorus concentrations of these known standards. The actual phosphorus concentrations of the standards are typically measured by chemical analysis techniques.
An improvement on these basic infrared spectroscopic methods deals with performing a correction to the calibration curve as disclosed in A. S. Tenney and M. Ghezzo, "Composition of Phosphosilicate Glass by IR Absorption," 120 Journal of the Electrochemical Society 1276 (1973). Specifically, the amplitude ratio calculated from the band amplitudes is recalculated using the ratio between the P.dbd.O and Si--O band areas rather than their band amplitudes. The advantage in employing the band area ratio in lieu of the linear amplitude ratio is that the band area ratio appears to reduce the temperature dependence in the calibration curve. This temperature dependence is more prevalent in BPSG films than in PSG films.
Unfortunately, these previous methods for measuring the phosphorus concentration in PSG films have not been as accurate as required, and the detection limits afforded by these previous methods have not been very low. These previous methods have failed to address and overcome several problems inherent in measuring the phosphorus concentration in PSG films.
Most importantly, these previous methods have failed to address and overcome the inherent problem of overlapping bands in the PSG infrared spectrograph. The problem associated with overlapping bands in PSG films is well known and is generally disclosed in the articles cited above and in K. Nassau, R. A. Levy and D. L. Chadwick, "Modified PSG's for VLSI Application," 130 J. Electrochemical Societ 404 (February 1985). Specifically, previous methods have not dealt effectively with the overlap of the P.dbd.O band at about 1316 cm.sup.-1 and the Si--O stretching band whose maximum is at about 1080 cm.sup.-1. The overlap problem results in not being able to measure accurately the amplitude of the P.dbd.O band for use in quantitative analysis. This overlap has been the main impediment to measuring the phosphorus concentration in PSG films.
Measuring the phosphorus concentration in BPSG films with the infrared spectroscopic method is additionally more complex because of the presence of boron. Some previous methods of measuring the phosphorus concentration in BPSG films involve using the uncorrected amplitude measurements at about 1316 cm.sup.-1 for the phosphorus analysis and at about 1370 cm.sup.-1 for the boron analysis to derive calibration curves. Other methods have used a procedure similar to that discussed above for analysis of PSG films in which an amplitude ratio is calculated by first measuring the amplitudes of two relevant bands on the spectrum and then dividing one of the amplitudes by the other to normalize for film thickness. See generally, W. Kern and G. L. Schnable, "Chemically Vapor-Deposited BPSG for Silicon Device Application," RCA Review 43, p. 423 (September 1982). A calibration curve is then constructed by determining the amplitude ratio for samples used as standards and plotting these ratios against the corresponding the actual phosphorus concentration of the standards as typically measured by chemical analysis techniques.
Like previous methods of measuring the phosphorus concentration in PSG films, previous methods of measuring the phosphorus concentration in BPSG films have not been as accurate as required, and detection limits have not been adequately very low. These previous methods have likewise failed to address and overcome several problems inherent in measuring the phosphorus concentration in BPSG films.
Most importantly, these previous methods have failed to address and overcome the inherent problem of overlap between the P.dbd.O and B--O absorption bands at about 1316 and at about 1370 cm.sup.-1 respectively. The overlap problem in BPSG films is generally disclosed in F. S. Becker and D. Pawlik, "A New LECVD BPSG Process Based on the Doped Deposition of TEOS-Oxide," Proceedings of the Symposium of Reduced Temperature Processing for VLSI, R. Reif and G. R. Srinivason, eds., Electrochemical Society (1986). This overlap in BPSG films is in addition to the overlap of the P.dbd.O band at about 1316 cm.sup.-1 and the Si--O band at about 1080 cm.sup.-1 Both sets of overlapping bands have been the main impediment to measuring the phosphorus concentration in BPSG films. Besides this overlap, the phosphorus band is weak in comparison to the boron band, which tends to obscure the phosphorus band.
Furthermore, as revealed in the article by F. S. Becker, D. Pawlik, H. Schafer and G. Staudigl, "Process and Film Characterization of Low Pressure Tetraethylorthosilicate Borophosphosilicate Glass", J. Vac. Sci. Technol. B 4(3), 732 May/June 1986, difficulties have been experienced in calibration, i.e., finding a quantitative relationship between spectral features and the boron concentrations in BPSG films. In addition, the strongest absorption of the silicon-oxygen vibration at about 1080 cm.sup.-1 changes band form and position of the band maximum depending on the dopant concentration. Even the band at 450 cm.sup.-1, corresponding to another silicon-oxygen vibration, is broadened when boron is one of the glass components. These difficulties have been addressed and overcome by the present invention.
Various methods have been used to resolve close or overlapping bands in infrared spectroscopy. The preferred embodiment of the present invention uses the derivative spectroscopic technique. Derivative spectroscopy has been disclosed in J. E. Cahill, "Derivative Spectroscopy: Understanding Its Application," American Laboratory, November, 1979, p. 79. Derivative spectroscopy calculates the first, second or higher order derivative of a spectrum with respect to wavelength or frequency. As each higher order derivative is calculated, each of the spectrograph's bands become more defined individually, and close or overlapping bands become more resolved. The derivative plot is used for further analysis, rather than the spectrum itself. Amplitudes are measured from the derivative plot rather than the original plot spectral curve.
The application of derivative spectroscopy to infrared spectroscopic methods has been limited. Derivative spectroscopy has been used in a procedure different from the present invention, in G. L. Collier and F. Singleton, "Infrared Analyses by the Derivative Method," 6 J. Appl. Chem. 495 (November 1956), and G. L. Collier and A. C. M. Panting, "The Use of Derivative Spectroscopy for Determining Methyl Groups in Polythene," 14 Spectrochemical Acta 104 (1959).
As discussed above, inherent problems associated with poor resolution of overlapping bands in infrared spectrographs of phosphorus in PSG and BPSG films and the limited sensitivity of these techniques to low concentrations of phosphorus are recognized drawbacks. Furthermore, as typical of all other analytical techniques which yield relative results, infrared spectroscopic techniques require proper standardization, a task hard to achieve with previous infrared spectroscopic techniques.
The present invention addresses and overcomes the problem of overlapping bands and changing band forms in measuring the phosphorus concentration in PSG and BPSG films through the use of derivative spectroscopy. Derivative spectroscopic techniques are used to separate and resolve each of the bands in the spectrograph so that the influence of overlapping bands on each other will be minimized.
The resolution of overlapping bands also lowers the sensitivity of the technique since the influence from other bands is minimized. The present invention then increases the sensitivity of previous methods to detect low concentrations of phosphorus.
Further, the use of the described amplitude ratio obviates the need to monitor film thicknesses, and the use of a calibration curve based on these ratios provides standardization for the present invention. This standardization is advantageous if the process is to be used in quality control applications.
Moreover, for BPSG films, the dependence of the phosphorous concentration on the boron concentration is incorporated into the calibration curve.
In general, the infrared techniques have advantages over other techniques because the instrumentation costs are less on a relative scale, it is nondestructive, fast, accurate, and it can be performed by relatively unskilled operators.
It is accordingly a general object of the invention to provide greater accuracy and a lower detection limit in both PSG and BPSG films by making it possible to deal effectively with the overlap of the P.dbd.O band at about 1316 cm.sup.-1 and the Si--O stretching band whose maximum is at about 1080 cm.sup.-1.
For BPSG films, the present invention further provides a means to deal effectively with the additional overlap of the P.dbd.O band, whose maximum is at about 1316 cm.sup.-1, and the B--O band whose maximum occurs at about 1370 cm.sup.-1
This invention finds application by semiconductor manufacturers for process monitoring and development activities. Furthermore, manufacturers of infrared spectrometers might include this procedure as a software package and application and utilize it to sell not only the software but also their instrumentation.